The topic of World War I came up last night with a few strangers, at a bar, of course. One seemingly knowledgeable gentleman, who happened to have Howard Zinn's A Peoples' History of the United States in his bag, suggested that America entered World War I solely because England owed JP Morgan a large sum of money. By sending troops to Europe, America was, in effect, protecting its investment. I suggested there was more to it than just that: isolationist guilt, perhaps.

In most wars, there seems to be a clear reason for America's participation. What was the main reason for US participation in World War I?

I love me some Zinn, but frankly that's more of a conspiracy theory (probably well suited to bars) than a real theory. I'll try to post an answer, but I want to wait till I can get home and consult my copy to see what Zinn actually said.
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T.E.D.♦Jul 13 '12 at 14:57

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BTW: Did he get her number? :-) www.youtube.com/watch?v=ymsHLkB8u3s
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T.E.D.♦Jul 13 '12 at 15:02

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@HermannIngjaldsson - I never found the reasons for either particularly unclear. A really bad idea in both cases perhaps, but not all that unclear. Then again, as an actual American alive during both, perhaps I have insights into our thinking that those outside our borders do not share.
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T.E.D.♦Jul 13 '12 at 15:40

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@HermannIngjaldsson - those may have been pretenses, but I'd be careful to call them reasons.
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samthebrandJul 13 '12 at 15:45

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@HermannIngjaldsson - a great deal of people not in any way related to GWB (including possibly Saddam himself) firmly believed that Iraq had WMDs.
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DVKJul 21 '12 at 9:47

The book talks about how once the war got underway in 1914 after a short period of non-involvement the war quickly became "the biggest profit-making enterprise in the history of American exporting" for American enterprises. This included farmers, and bankers. In fact the Allies borrowed $2.5 billion from the Americans, whereas the Germans only borrowed $45 million. I'm sure JP Morgan was a part of those loans in some way. The book American Foreign Relations: A History, Vol. 2: Since 1895 tells much the same story as For the Common Defense. As American Foreign Relations put it on page 76 "[n]eutral or not the United States had become the arsenal of the Allied war effort."

The US had a very strong economic self-interest in the Allies winning the war. Then of course there was the public outrage over the ever-increasing amount of German U-boat attacks, as most publicized in the Lusitania incident. Of course the Germans didn't have much of a choice knowing full well that much of the American shipping they were sinking was probably carrying supplies to the Allies. There was also the Zimmerman telegram incident, but at that point the die had already been cast.

Additionally, prior to World War I the US was in the process of becoming a world power and flexing its economic and military muscle. It had acquired colonies (Puerto Rico, Cuba, American Samoa, Guam, Philippines) and bested the former Spanish Empire in the Spanish-American War, which resulted in the acquisition of all but American Samoa. The country was probably in the right psychological mindset to engage in the Great War.

Economic self-interest was the key factor in the US entering WWI. That self-interest was paired with the actions of Germany (U-boats, and Zimmerman telegram), a growing sense of world-powerness (fueled by colonial growth and economic strength), and a dislike for autocratic systems of government.

Upvoted this, but I strongly dispute the "didn't have much choice" part. The Germans actually backed off unrestricted sub warfare for quite a while specifically because they knew how the Americans would react. They only resumed it when events in the east, and their own local u-boat proponents, convinced the leadership that they could defeat the Allies this way before the USA would have time to declare war and come over in any significant numbers. Sadly for them, the Brits countered with the convoy system, and the (predicted) USA response lost them the war.
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T.E.D.♦Jul 13 '12 at 18:38

@T.E.D.: In fact, the US involvement had very little effect on the war. Germany would have lost anyway. This is partly because the US military leadership refused to listen to the European leaders, trying to tell them that 19th century tactics didn't work anymore (but to their credit they learnt their lesson and started being more useful at the end, the French and British generals took years to learn that lesson). But most importantly, by summer 1918, when US troops arrived, Germany was in disarray. That said, the entry of the Americans may have had a great psychological effect.
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Lennart RegebroJul 22 '12 at 15:50

The U.S. entered World War I because Germany (needlessly) DIRECTLY threatened U.S. interests.

The sinking of the Lusitania, with the loss of 128 American lives caused a lot of ill-will in the United States. And the resulting "unrestricted submarine" warfare was a threat to American notions of free trade going back at least to the War of 1812.

But the "last straw" was the Zimmermann Telegram, sent to "Mexico" proposing a "stab in the back."

Mexico was in the throes of a civil war, and really had no central government. The Germans believed otherwise because one of the four major factions (under Pancho Villa) "invaded" the U.S. while fleeing from the others.

The U.S. would be threatened by a world in which Germany occupied Belgium and northern France (leaving a "rump" state in the south, as in World War II), probably northern Italy, and dominated the Balkans, Scandinavia, the Middle East, and the Baltic and East European countries near the Russian border, probably after uniting with Austria-Hungary (who had lost the heir to the throne when Archduke Franz Ferdinand was assassinated).

To have such a country allied with Mexico (or Brazil or Argentina) was too much, and a clear violation of the Monroe Doctrine.